build(deps): bump DeterminateSystems/update-flake-lock from 20 to 21

Bumps [DeterminateSystems/update-flake-lock](https://github.com/determinatesystems/update-flake-lock) from 20 to 21.
- [Release notes](https://github.com/determinatesystems/update-flake-lock/releases)
- [Commits](DeterminateSystems/update-flake-lock@v20...v21)

---
updated-dependencies:
- dependency-name: DeterminateSystems/update-flake-lock
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/update-flake-lock.yml

@@ -14,4 +14,4 @@ jobs:
         with:
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
-      - uses: DeterminateSystems/update-flake-lock@v20
+      - uses: DeterminateSystems/update-flake-lock@v21